feat: add structured economic events

tests/npc_visual_lifecycle_test.gd

@@ -18,6 +18,24 @@ func _run() -> void:
 	var npc: SimNPC = manager.npcs[0]
 	var visual: Node3D = view.active_npc_visuals[npc.id]
 	visual.set_physics_process(false)
+	var carried_food: MeshInstance3D = visual.get_node("CarriedFood")
+	_check(not carried_food.visible, "NPC should begin without a carried-food prop")
+	npc.add_inventory(SimulationIds.RESOURCE_FOOD, 1.0)
+	manager.emit_signal(
+		"npc_inventory_changed",
+		npc,
+		SimulationIds.RESOURCE_FOOD,
+		1.0
+	)
+	_check(carried_food.visible, "Carried food should be visible on the NPC")
+	npc.remove_inventory(SimulationIds.RESOURCE_FOOD, 1.0)
+	manager.emit_signal(
+		"npc_inventory_changed",
+		npc,
+		SimulationIds.RESOURCE_FOOD,
+		0.0
+	)
+	_check(not carried_food.visible, "Deposited or eaten food should be hidden")
 
 	var resource_state: ResourceStateRecord = manager.get_resource_state(
 		&"berry_bush_01"